Agree with bro Qxn... if u change to 4 x 4ft T5... u can already keep proper clams. Since u have very narrow tank, the Croceas and Maximas can attach nicely to the rockscape. My clams look healtier on my rockscape... the sand sometimes irritate them coz of moving particles and they sometimes lean to one side.
Clams are definitely a must for ur tank!!! But don't pay more than $25 for one ok? Those $80 each is still questionable... whether worth investing as a first clam.